Shigellosis is a leading cause of bacillary dysentery in humans. Each year, over 163 million cases occur worldwide, with the majority of cases occurring in children in developing countries, and 0.0 million cases resulting in death. Antibiotics are generally effective against shigellosis, but because Shigellae are increasingly developing antibiotic resistance, even to the newest antibiotics, the World Health Organization has given priority to the development of a safe and effective vaccine against Shigella. 
Four Shigella species (or groups) are now recognized: S. dysenteriae (group A), which has 15 serotypes; Shigella flexneri (group B), which has 14 classical serotypes and subserotypes; Shigella boydii (group C), which has 20 serotypes; and Shigella sonnei (group D), which has a single serotype. The target populations for the use of Shigella vaccines include infants and young children in developing countries (in whom the peak incidence occurs at 12-47 months of age and the S. flexneri serotypes predominate). S. Dysenteriae 1, which produces Shiga toxin and typically carries R factors that encode resistance to multiple antibiotics, causes epidemics of this disease worldwide.
S. sonnei persists in developed (and transitional) countries, causing sporadic diarrhea and occasional outbreaks in epidemiological niches. Travelers from developed to developing regions, who mainly acquire S. sonnei and S. flexneri infections, represent another target population for Shigella vaccines. Consequently, a Shigella vaccine that can provide a high level of protection against S. dysenteriae 1, all S. flexneri serotypes and S. sonnei would constitute an epidemiological valid ‘global’ vaccine but to include such large number of serotypes is not feasible. Three main serotypes, S. Sonnei, S. flexneri 2a and S. flexneri 6, caused 79% of the cases of shigellosis so a combination of these three with epidemic dysentery causing S. dysenteriae 1 wall serve the purpose.
Convincing evidence that an initial clinical Shigella infection elicits serotype-homologous protection comes from three sources: NHP challenge studies, volunteer model re-challenge studies and prospective epidemiological surveillance of a cohort of children in an endemic area. Many approaches have been used for Shigella vaccines such as live attenuated, killed whole bacteria, Shigella LPS or O-polysaccharide conjugated to carriers such as proteosomes, tetanus toxoid and ribosomes. Inspite of extensive research for so many years an effective Shigella vaccine is still not available and the greatest impediment to achieving a useful Shigella vaccine is devising a strategy that can confer broad protection against a large number of epidemiologically relevant serotypes.